Biomimetic Hybrid Actuators employed in biologically-inspired musculoskeletal architectures as described in the above noted U.S. patent application Ser. No. 11/395,448 employ an electric motor for supplying positive energy to and storing negative energy from an artificial joint or limb, as well as elastic elements such as springs, and controllable variable damper components, for passively storing and releasing energy and providing adaptive stiffness to accommodate level ground walking as well as movement on stairs and surfaces having different slopes.
The above noted application Ser. No. 11/495,140 describes an artificial foot and ankle joint consisting of a curved leaf spring foot member that defines a heel extremity and a toe extremity, and a flexible elastic ankle member that connects said foot member for rotation at the ankle joint. An actuator motor applies torque to the ankle joint to orient the foot when it is not in contact with the support surface and to store energy in a catapult spring that is released along with the energy stored in the leaf spring to propel the wearer forward. A ribbon clutch prevents the foot member from rotating in one direction beyond a predetermined limit position, and a controllable damper is employed to lock the ankle joint or to absorb mechanical energy as needed. The controller and a sensing mechanisms control both the actuator motor and the controllable damper at different times during the walking cycle for level walking, stair ascent and stair descent.
The above noted U.S. patent application Ser. No. 11/600,291 describes an exoskeleton worn by a human user consisting of a rigid pelvic harness worn about the waist of the user and exoskeleton leg structures each of which extends downwardly alongside one of the human user's legs. The leg structures include hip, knee and ankle joints connected by adjustable length thigh and shin members. The hip joint that attaches the thigh structure to the pelvic harness includes a passive spring or an active actuator to assist in lifting the exoskeleton and said human user with respect to the ground surface upon which the user is walking and to propel the exoskeleton and human user forward. A controllable damper operatively arresting the movement of the knee joint at controllable times during the walking cycle, and spring located at the ankle and foot member stores and releases energy during walking.
The additional references listed below identify materials which are referred to in the description that follows. When cited, each reference is identified by a single number in brackets; for example, the first reference below is cited using the notation “{1}.”    1. Palmer, Michael. Sagittal Plane Characterization of Normal Human Ankle Function across a Range of Walking Gait Speeds. Massachusetts Institute of Technology Master's Thesis, 2002.    2. Gates Deanna H., Characterizing ankle function during stair ascent, descent, and level walking for ankle prosthesis and orthosis design. Master thesis, Boston University, 2004.    3. Hansen, A., Childress, D. Miff, S. Gard, S. and Mesplay, K., The human ankle during walking: implication for the design of biomimetic ankle prosthesis, Journal of Biomechanics, 2004 (In Press).    4. Koganezawa, K. and Kato, I., Control aspects of artificial leg, IFAC Control Aspects of Biomedical Engineering, 1987, pp. 71-85.    5. Herr H, Wilkenfeld A. User-Adaptive Control of a Magnetorheological Prosthetic Knee. Industrial Robot: An International Journal 2003; 30: 42-55.    6. Seymour Ron, Prosthetics and Orthotics: Lower limb and Spinal, Lippincott Williams & Wilkins, 2002.    7. G. A. Pratt and M. M. Williamson, “Series Elastic Actuators,” presented at 1995 IEEE/RSJ International Conference on Intelligent Robots and Systems, Pittsburgh, Pa., 1995.    8. Inman V T, Ralston H J, Todd F. Human walking Baltimore: Williams and Wilkins; 1981.    9. Hof. A. L. Geelen B. A., and Berg, J. W. Van den, “Calf muscle moment, work and efficiency in level walking; role of series elasticity,” Journal of Biomechanics, Vol 16, No. 7, pp. 523-537, 1983.    10. Gregoire, L., and et al, Role of mono- and bi-articular muscles in explosive movements, International Journal of Sports Medicine 5, 614-630.    11. Endo, K., Paluska D., Herr, H. A quasi-passive model of human leg function in level-ground walking IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS); Oct. 9-16, 2006; Beijing, China.
As noted in references {1}, {2}, {3}, and {4} above, an artificial limb system that mimics a biological limb ideally needs to fulfill a diverse set of requirements. The artificial system must be a reasonable weight and have a natural morphological shape, but still have an operational time between refueling or battery recharges of at least one full day. The system must also be capable of varying its position, stiffness, damping and nonconservative motive power in a comparable manner to that of a normal, healthy biological limb. Still further, the system must be adaptive, changing its characteristics given such environmental disturbances as walking speed and terrain variation. The current invention describes a novel actuator and limb architecture capable of achieving these many requirements.
From recent biomechanical studies described in references {1}, {2} and {3} above, researchers have determined that biological joints have a number of features. Among these are:    (a) the ability to vary stiffness and damping;    (b) the ability to generate large amounts of positive mechanical work (nonconservative motive output); and    (c) the ability to produce large amounts of power and torque when needed.
An example of the use of more than one control strategy in a single biological joint is the ankle. See {1} and {2}. For level ground ambulation, the ankle behaves as a variable stiffness device during the early to midstance period, storing and releasing impact energies. Throughout terminal stance, the ankle acts as a torque source to power the body forward. In distinction, the ankle varies damping rather than stiffness during the early stance period of stair descent. These biomechanical findings suggest that in order to mimic the actual behavior of a human joint or joints, stiffness, damping, and nonconservative motive power must be actively controlled in the context of an efficient, high cycle-life, quiet and cosmetic biomimetic limb system, be it for a prosthetic or orthotic device. This is also the case for a biomimetic robot limb since it will need to satisfy the same mechanical and physical laws as its biological counterpart, and will benefit from the same techniques for power and weight savings.
The current state of the art in prosthetic leg systems include a knee joint that can vary its damping via magnetorheological fluid as described in {5}, and a carbon fiber ankle which has no active control, but that can store energy in a spring structure for return at a later point in the gait cycle e.g. the Flex-Foot {4} or the Seattle-Lite {6}. None of these systems are able to add energy during the stride to help keep the body moving forward or to reduce impact losses at heel strike. In the case of legged robotic systems, the use of the Series Elastic Actuator (SEA) enables robotic joints to control their position and torque, such that energy may be added to the system as needed. See {7}. In addition, the SEA can emulate a physical spring or damper by applying torques based on the position or velocity of the joint. However, for most applications, the SEA requires a tremendous amount of electric power for its operation, resulting in a limited operational life or an overly large power supply. Robotic joint designs in general use purely active components and often do not conserve electrical power through the use of variable-stiffness and variable-damping devices.